central air conditioning installation

How much to purchase and install central air conditioning?
I have an 1800 sq ft. split level.
House already has ducting for gas forced air heating.
Can anyone give me a very rough guess as to how much I would be looking at for purchase and installation?
Rough guess, about $3000-$5000. They have recently changed the requirements for heating/cooling efficiency so everything has gone up but if you have the ducting in place it will cost less than a completely new installation.
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